Adds perf tests for `Map.merge`. (#1407)
Adds perf tests for `Map.merge`. (#1407)

* Adds perf tests for `Map.merge` and `Map.mergeDeep`.

* Per review, updated descriptions, and test merges different Map instances (and content).

* Ensure merge has differing values during perf test

* Remove mergeDeep()

It wasn't actually running mergeDeep() and as a perf test, it's a thin composition atop merge() anyhow.

facebook/immutable-js,facebook/immutable-js,facebook/immutable-js,facebook/immutable-js